1. Field of the Invention
The present invention relates to metallothermic processes for producing metallic chromium and its alloys. More specifically, the present invention relates to metallothermic processes for producing low-nitrogen metallic chromium and chromium-containing alloys and to the products obtained by said processes.
2. Description of Related Art
The lifespan of rotating metal parts in aircraft engines is typically determined by fatigue cracking. In this process, cracks are initiated at certain nucleation sites within the metal and propagate at a rate related to the material characteristics and the stress to which the component is subjected. That, in turn, limits the number of cycles the part will withstand during its service life.
Clean melting production techniques developed for superalloys have given rise to the substantial elimination of oxide inclusions in such alloys to the extent that nowadays, fatigue cracks are mainly originated on structural features, for example, on grain boundaries or clusters of primary precipitates such as carbides and nitrides.
It has been found that the primary nitride particles formed during the solidification of alloy 718 (see alloy 718 specifications (AMS 5662 and API 6A 718))—which is one of the main alloys utilized in the production of aircraft engine rotating parts and for oil and gas drilling and production equipment—are pure TiN (titanium nitride) and that the precipitation of primary Nb—TiC (niobium-titanium carbide) occurs by heterogeneous nucleation over the surface of the TiN particles, thereby increasing the precipitate particle size. The particle size can be decreased by two means: either by lowering the carbon content as much as possible, or by lowering the nitrogen content.
Many commercial specifications for stainless steel, other specialty steels, and superalloys, establish minimum carbon content, usually in order to prevent grain boundary slipping at the service temperature. As a consequence, the only practical means to decrease particle size compositionally is to reduce the nitrogen content in the material as extensively as possible. In that way, in as much as the nitrides precipitate first, removing nitrogen supersedes the importance of removing carbon.
It is known that removing the nitrogen and/or the nitrogen-containing precipitates after the reduction of a metal or metal alloy is an extremely difficult and expensive task. Therefore, nitrogen preferably should be removed before or during the reduction process.
There is a well known process for producing low nitrogen alloys called electron beam melting; it is very expensive and extremely slow when compared to a metallothermic reduction process and therefore, impractical from a commercial point of view. There is also a known aluminothermic reduction process (see, U.S. Pat. No. 4,331,475) which, as opposed to embodiments of the present invention, is not conducted under continuous reduced pressure resulting, at best, in a chromium master alloy, with a reduced nitrogen content of 18 ppm which, when used in alloy 718 production, cannot guarantee an alloy 718 whose nitrogen content is below the solubility limit of the titanium nitride precipitate.